1. Field of the Invention
The invention relates to an encased piezoelectric actuator having an encased strain generating element which generates strain due to the piezoelectric effect or the electrostriction effect, and more particularly to an encased piezoelectric actuator having a preloaded strain generating element.
2. Description of the Related Art
As is known in the art, a piezoelectric actuator converts an electric energy into a mechanical energy such as an elongation and a force by virtue of the piezoelectric effect. A conventional piezoelectric actuator will be explained hereinbelow with reference to FIG. 1. The following description concerns an actuator having an electrostriction effect element, however, it should be noted that the term "a piezoelectric actuator" herein is defined to include both an actuator having a piezoelectric effect element as a strain generating element and an actuator having an electrostriction effect element as a strain generating element. Accordingly, the terms "piezoelectric" and "electrostriction" may be interchanged with each other in the following disclosure.
In general, an electrostriction effect element used as a strain generating element of a piezoelectric actuator is often made of ceramics, because ceramics show the electrostriction effect. However, ceramics are brittle against mechanical impacts, and are weak against a tensile force, though ceramics are resistant to compressive forces. Accordingly, a piezoelectric actuator conventionally has been accommodated in a metal case to protect it from mechanical impacts, and further preloaded or given a compression force in advance so as to offset a tensile force acting on the piezoelectric actuator.
Another object of encasing a strain generating element is to enhance durability against the environment. That is, by hermetically sealing a case, it is possible to operate a piezoelectric actuator even in a high humidity atmosphere or a corrosive gas atmosphere to thereby greatly enhance the reliability of a piezoelectric actuator. This advantageous effect is significant to an actuator having a laminated type element and an electrostriction thin film ceramic layer to which a high magnitude of electric field is to be applied.
Another object of preloading a strain generating element is to control an elongation of an actuator utilizing a well-known phenomenon that a strain of a strain generating element to which a driver voltage is applied is dependent on a compression force acting on the element.
In an actuator illustrated in FIG. 1, an electrostriction effect element 1 operating as a strain generating element is encased and hermetically sealed in a cylindrical metal case 4 which is made of stainless steel, for instance. The electrostriction effect element 1 is of a laminated type and has a property of generating a large mechanical energy by a low drive voltage. A length of the metal case 4 is determined in accordance with a length of the element 1 to be encased in the case 4, and a length of the element 1 is determined in accordance with a required elongation.
As illustrated in FIG. 1, to the electrostriction effect element 1 is adhesively secured a diaphragm 3 and a bottom plate 19 both made of a metal. The diaphragm 3, the bottom plate 19 and the metal case 4 are hermetically welded to each other at their junctions. Lead wires 2a and 2b of the electrostriction effect element 1 are electrically connected to lead terminals 5 provided on the metal case 4.
On a peripheral edge of the diaphragm 3 is welded a cylindrical metal pipe 6. In the metal pipe 6 is placed an elongation transferring device 7 having a projection 7a extending oppositely to the element 1. The elongation transferring device 7 is formed at an outer circumferential surface thereof with a groove. An O-ring 8 is interposed between the groove and an internal surface of the metal pipe 6. The O-ring 8 guides the device 7 along the internal surface of the metal pipe 6 when the device 7 generates an elongation. If the device 7 are to be integrally formed with the diaphragm 3, it is not always necessary to provide the O-ring 8.
The above mentioned encased piezoelectric actuator is individually used. This causes problems that it is difficult to simplify parts control in manufacturing steps and decrease the number of manufacturing steps, and that an actuator is lacking in the flexibility for wide use or a wide range of elongations. Hereinbelow will be explained in detail.
As aforementioned, an elongation of an electrostriction effect element is dependent on a length of the element along a direction in which the element generates an elongation, under otherwise unchanged conditions. That is, in principle, the length of a strain generating element is without limit in accordance with various uses of a piezoelectric actuator, and accordingly the length of a metal case for encasing the element is also without limit. When an actuator is to be sold, actuators are classified in accordance with their elongations into several groups. Thus, the length of the element is much limited, however, it is still required to prepare several lengths of the elements. For instance, there has been prepared at least three length, e.g., 20 mm, 40 mm and 60 mm, for the laminated type electrostriction effect element 1 used for a conventional actuator illustrated in FIG. 1. Accordingly, it is necessary to prepare at least three lengths of the metal case 4. Thus, when a conventional actuator is intended to be used to obtain a variety of elongations, it is not possible to avoid the increase of the number of parts and the complexity of parts control.
When elements each having length of 40 mm or 60 mm are to be formed, a plurality of unit elements having a length of 20 mm are secured to each other in accordance with a desired length. However, as the number of a required length of an element is increased, element manufacturing steps are increased accordingly and also the number of necessary jigs are increased. In addition, if a manufacturing plan has to be changed, it is difficult to absorb such a change in the manufacturing steps.